TiO.sub.2, V.sub.2 O.sub.5, ZnO, WO.sub.3, etc. have heretofore been known as substances which, when irradiated by ultraviolet radiation, cause oxygen molecules to be adsorbed to or desorbed from an organic compound such as a smelly constituent for promoting decomposition (oxidation) of the organic compound. Since particles of TiO.sub.2 whose crystallized form is anatase, in particular, are highly effective for use as a photocatalyst, it has been proposed to form a photocatalytic layer of TiO.sub.2 particles on the surface of walls, tiles, glass (mirror), circulatory filter units, or sanitary ware.
Known processes of depositing such a photocatalytic layer of TiO.sub.2 particles directly on the surface of a base of plastic, ceramic, or resin include the CVD process, the sputtering process, and the electron beam evaporation process.
However, the CVD process, the sputtering process, and the electron beam evaporation process require large-scale equipment, and result in a high manufacturing cost due to a poor yield.
According to other known processes of forming a photocatalytic layer, photocatalytic particles are kneaded with a binder and coated on the surface of a base by spray coating or dip coating, and thereafter heated (see Japanese laid-open patent publication No. 5-201747).
For photocatalytic particles such as TiO.sub.2 particles to function effectively as a photocatalyst, it is necessary that the photocatalytic particles be irradiated with ultraviolet radiation and held in contact with a substance to be decomposed thereby such as a smelly gas or the like. Kneading photocatalytic particles with a binder and coating them on the surface of a base as disclosed in Japanese laid-open patent publication No. 5-201747 tends to embed many photocatalytic particles in the binder layer, so that no ultraviolet radiation will reach and no smelly gas or the like will contact those photocatalytic particles, which thus will fail to perform a sufficient photocatalytic function.
Another known process of forming a photocatalytic layer is an alkoxide process disclosed in Japanese laid-open utility model publication No. 5-7394. According to the disclosed process, a photocatalytic layer is formed by coating a base of glass with a titanium alkoxide, drying the coated titanium alkoxide, and thereafter firing the titanium alkoxide at 100.degree. C. An organic material in water is decomposed when an ultraviolet radiation is applied to the photocatalytic layer.
The alkoxide process is excellent in that it can form a thin film at a relatively low temperature, and is effective where a substance such as Pyrex glass or quartz glass which is not softened at temperatures up to about 500.degree. C. is used as the material of the base. If a substance such as soda glass having a low melting point is used as the material of a base, then the base is softened already at a temperature at which a thin film is formed, and a formed thin photocatalytic film is embedded in the base, with the result that no light will reach the photocatalytic layer, which will fail to perform photocatalytic functions.
According to Japanese laid-open patent publication No. 1-288321, a TiO.sub.2 sol is sprayed over ceramic paper of a fibrous material and thereafter heated at a temperature ranging from 400 to 700.degree. C., and thereafter an SnO.sub.2 sol is sprayed and heated at a temperature ranging from 400 to 700.degree. C., thus forming a photocatalytic film capable of oxidizing and decomposing aldehydes.
In the process disclosed in Japanese laid-open patent publication No. 1-288321, the entire surface of the film is covered with SnO.sub.2 which is less active than TiO.sub.2, and attempts to increase the strength of the film are liable to cause cracks. Specifically, as shown in FIG. 1(a) of the accompanying drawings, when a sol containing TiO.sub.2 particles 101 is coated on the surface of a tile 100 and then heated (fired), a crack 102 develops as shown in FIG. 1(b) of the accompanying drawings. The crack is considered to occur because a phase transition to a rutile structure brings about a volume shrinkage (a density increase) and also because whereas the distance between TiO.sub.2 particles 101 is L.sub.0 as shown in FIG. 2(a) of the accompanying drawings before being fired, the distance is reduced to L.sub.1 (L.sub.1 &lt;L.sub.0) as shown in FIG. 2(b) of the accompanying drawings due to volumetric diffusion of the particles into each other after being fired into the rutile structure.
Japanese patent publication No. 4-46609 discloses a method of purifying an odor in the air in the passenger's compartment of a vehicle by decomposing or modifying a smelly substance contained in the odor. Specifically, light is applied to a solid semiconductor photocatalyst in which a metal or a metal oxide is carried by a semiconductor, and the air to be purified in the passenger compartment is brought into contact with the solid semiconductor photocatalyst for decomposing or modifying the smelly substance contained in the odor due to a photochemical reaction.
However, when a base with its surface coated with a photocatalyst is used in contaminated water or an outer wall, dirt tends to be deposited on the photocatalyst owing to high-molecular substances, dust particles, and bacteria that are contained in the atmospheric air or water. Depending on the type of the dirt, the photocatalytic function may be lowered by the deposited dirt.
One solution to the reduction in the photocatalytic function due to deposited dirt is disclosed in Japanese patent publication No. 6-7905. Japanese patent publication No. 6-7905 reveals a deodorizing apparatus comprising a photocatalytic layer of semiconductor, an ultraviolet lamp and a heater disposed in confronting relation to the photocatalytic layer, and an air blower. The photocatalytic layer or the heater, or the photocatalytic layer and the heater are moved for sequentially heating the photocatalytic layer in its entirety. When the photocatalytic layer is heated to a temperature close to 400.degree. C., dirt of high-molecular substances, dust particles, and the like is removed, and the photocatalytic layer is regenerated.
The above arrangement of regenerating the photocatalytic layer is practically difficult to use on a member of an installation disposed inside a room. Therefore, a more radical solution is desired not to remove dirt deposited on the photocatalytic layer but to prevent dirt from being deposited on a photocatalytic layer or to prevent a photocatalytic layer from lowering its photocatalytic function even with dirt deposited thereon.
Japanese patent publication No. 6-7906 shows a process of applying light with high ultraviolet intensity to a photocatalyst to remove an odor in a home or an office. With this process of applying light with high ultraviolet intensity, however, the odor is decomposed at a different rate depending on the structure of the photocatalyst. Furthermore, though the base which carries the photocatalyst has sufficient mechanical strength if the base is porous and impregnated with the photocatalyst according to the process disclosed in the publication, no sufficient mechanical strength will be achieved if the base is made of a material which is not porous such as glazed tile or relatively dense ceramic.
TiO.sub.2 is available in different crystallized forms including anatase, brucite, and rutile. The anatase structure is better in photoactivity than the other crystallized forms. It has been reported in the magazine "Surface", 1987, vol. 25 that the photoactivity of rutile TiO.sub.2 is improved by carrying a metal such as Pt, Ag, or the like. However, rutile TiO.sub.2 does not have sufficient odor removal rate, density, and adhesiveness. If Ag or AgO is used as a metal to be added, then the resultant photocatalyst is not suitable for use on tiles and building materials because these metals are black.
A sol of titanium oxide produced by the hydrothermal method or the sulfuric acid method can easily be coagulated because it is composed of ultrafine particles. If the coagulated material is coated on the surface of a base, then it will cause gloss irregularities and cracks. For preventing a sol of titanium oxide from being coagulated, it has heretofore been customary to attach an organic dispersant such as of triethanolamine to the surface of the titanium oxide sol.
If a monodisperse titanium oxide sol with an organic dispersant such as of triethanolamine being attached to the surface of the titanium oxide sol is coated on a base such as a resin base of low heat resistance and then fired and secured at 300.degree. C., then since the organic dispersant is firmly fixed to the active sites of the titanium oxide sol and is not sufficiently evaporated and decomposed in the firing step, the resultant material exhibits an insufficient photocatalytic action, and has an insufficient deodorizing and antibacterial capabilities.
Japanese laid-open patent publication No. 5-253544 discloses a process of kneading anatase titanium oxide with a binder, coating the mixture on the surface of a base, and heating the coated base. According to the disclosed process, a binder layer is formed on the surface of a plate member serving as a wall surface, a floor surface, or a ceiling surface of a dwelling space, and then a fine powder of photocatalyst made primarily of anatase titanium oxide is sprayed onto the surface of the binder layer such that the fine powder of photocatalyst is partly exposed from the binder layer. Thereafter, the binder layer is melted by being heated at a temperature of 300.degree. C. or higher and lower than 900.degree. C., after which the binder layer is cooled and solidified.
The photocatalytic material produced according to this method has a good deodorizing ability if heat-treated at a temperature of 300.degree. C. or higher and lower than 900.degree. C., but does not exhibit good deodorizing characteristics if heat treated at low temperatures lower than 300.degree. C. Therefore, it has been difficult to add a good photocatalytic activity such as excellent deodorizing characteristics to a base of plastic which is not resistant to heat. The reasons for this are considered to be as follows: In order to coat photocatalytic particles uniformly on a base, it is necessary to produce, in a preceding step, a monodisperse system of such photocatalytic particles dispersed in the suspension with an organic dispersant added. At temperatures lower than 300.degree. C., the added organic dispersant is not sufficiently decomposed and vaporized and remains in covering relation to active sites on the photocatalytic particles.
Therefore, it is an object of the present invention to provide a multi-functional material in which a photocatalytic layer is exposed from a base to exhibit a sufficient photocatalytic effect, and the base well retains the photocatalytic layer.
Another object of the present invention is to form a photocatalytic layer which is less peelable on a relatively dense base of glass, tile, metal, or plastic in comparison to conventional photocatalytic layers.
Still another object of the present invention is to form a photocatalytic layer on a base of relatively low melting point material, e.g., a base of soda glass which is relatively inexpensive and can easily be processed.
Yet still another object of the present invention is to provide a multi-functional material which is resistant to deposition of dirt thereon, has an antibacterial or deodorizing capability that is resistant to being lowered due to dirt, and has excellent mechanical strength.
A further object of the present invention is to provide a multi-functional material in which a photocatalytic layer made principally of anatase TiO.sub.2 has an excellent peel strength.
A still further object of the present invention is to increase the photocatalytic activity of a photocatalytic layer which is made primarily of rutile TiO.sub.2.
A yet still further object of the present invention is to increase the photocatalytic activity of a photocatalytic layer which is made primarily of rutile TiO.sub.2 and decolorize the photocatalytic layer for better appearance by having the photocatalytic layer carry Ag.
Still another object of the present invention is to provide a multi-functional material having a good photocatalytic function even if heat treated at low temperatures lower than 300.degree. C.